Polycarbonate is a colorless thermoplastic polymer, i.e., polycarbonate softens when heated and hardens when cooled. Polycarbonate is commonly used in applications which take advantage of its outstanding impact resistance and toughness, such as molded helmets, battery cases, bottles and packaging, and in applications which also demand optical transparency, such as bullet-proof and safety glass, eyewear, compact discs and automobile lenses. In thin-film form, polycarbonate is used for a variety of applications ranging from precision filters to electron-emitting devices.
Polycarbonate membranes used as commercial filters are described in the 1990 Nucleopore.RTM. Laboratory Products Catalog, Costar Corp., 1990, pp. 3, 8 and 9. The membranes are created by subjecting stretched, crystalline polycarbonate film to irradiation, followed by etching to form pores. The Costar process is similar to that disclosed in Price et al., U.S. Pat. No. 3,303,085. The thickness of commercial membrane filters is typically 6 to 11 .mu.m.
Bassiere et al., PCT Patent Publication WO 94/28569, disclose how thin polycarbonate layers are used in manufacturing electron-emitting devices. In one embodiment, Bassiere et al. provide a polycarbonate layer over a sandwich consisting of an upper conductor, an insulator and a patterned lower conductor. The multi-layer structure is irradiated with heavy ions to create radiation tracks through the polycarbonate layer. The tracks are etched to form pores through the polycarbonate layer down to the upper conductor. Using suitable etchants, the pore pattern in the polycarbonate layer is transferred to the upper conductor and then to the insulator, after which conical electron-emissive elements are formed in the resulting openings in the insulator.
Bassiere et al. indicate that the thickness of their polycarbonate layer is approximately 2 .mu.m. This is significantly less than the thickness of the commercial polycarbonate membrane filters in the Costar product catalog. While Bassiere et al. specify that the polycarbonate layer in their structure can be created by spin coating, Bassiere et al. do not provide any further information on how to make the polycarbonate layer.
Macaulay et al., PCT Patent Publication WO 95/07543, disclose a similar fabrication technique in which electron-emissive features in an electron-emitting device are defined by way of charged-particle tracks formed in a track layer. Polycarbonate is one of the materials that Macaulay et al. consider for the track layer. The thickness of the track layer in Macaulay et al. is 0.1 to 2 .mu.m, typically 1 .mu.m. Consequently, the thickness of the track layer in Macaulay et al. is typically less than that of the polycarbonate layer in Bassiere et al. by a factor of up to twenty.
As film thickness is reduced, it becomes progressively more difficult to make high-quality polycarbonate films. Controlling and maintaining the uniformity of film thickness and other properties, such as density, becomes harder. Structural and compositional defects also become more problematic in very thin polycarbonate films. In fabricating electron emitters, it would be desirable to have a process for making a thin polycarbonate film whose thickness and other physical properties are highly uniform, especially for film thicknesses in the range of 0.1 to 2 .mu.m.